Keeping two 3D Views in sync?
One way to get any number of 3D views to copy the view angle from one is to have all the 3D views in Camera view NumPad 0. Then check "Lock Camera To View" in the "View" section of the properties region N.

The downside to this approach is that you always have to stay in the camera view, you can not use any of the view navigation controls. However you can hold Alt while panning the view to snap the view at 45° angles.

The 3D view on the left is the view copying the center's view. The center is where I'm rotation the view, and where Lock Camera To View is checked. The 3D view on the right is just to show what is happening in the scene.